The present invention generally relates to image sensors, and more particularly to a contact type image sensor for optically reading a document image in a facsimile machine, for example.
Conventionally, an image sensor constituted by a one-dimensional charge coupled device (CCD) is used to read a document image in an optical apparatus such as a facsimile machine. However, the length of a photoelectric conversion part of the CCD is only in the order of 25 mm, for example, and a reduction system is inevitably required to optically read the document image. As a result, it is difficult to downsize the optical apparatus using such an image sensor because of the need to provide the reduction system.
In order to downsize the optical apparatus, it is necessary to use an image sensor which does not require the reduction system. Such an image sensor should at least have a length identical to the length of one side of the document which is to be read. When reading a document having A4 size, for example, the image sensor must have at least a length of 216 mm. An image sensor having the length of 216 mm cannot be produced by use of crystaline silicon, but it is possible to produce such an image sensor by use of a thin film made of an amorphous (non-crystaline) semiconductor such as amorphous silicon.
It is easy to produce an image sensor which can read a large document by using a semiconductor thin film as a photoelectric conversion layer because the length of the image sensor can easily be made identical to the length of one side of the document. For this reason, various kinds of contact type image sensors using the semiconductor thin film have been proposed. The contact type image sensor reads the document image by making direct contact with the document, and the optical apparatus using the contact type image sensor can be downsized since it is unnecessary to provide the reduction system.
The contact type image sensor generally has a sandwich structure or coplanar structure. The contact type image sensor having the sandwich structure has the semiconductor thin film sandwiched between an upper electrode and a lower electrode. On the other hand, the contact type image sensor having the coplanar structure has the electrodes extending from the sides of the semiconductor thin film on the same plane as the semiconductor thin film. The optical response characteristic of the contact type image sensor having the sandwich structure is better than that of the contact type image sensor having the coplanar structure because the film thickness of the semiconductor thin film can be made smaller in the sandwich structure.
FIG. 1 is a cross sectional view of the conventional contact type image sensor having the sandwich structure. In FIG. 1, the image sensor has a transparent dielectric substrate 1 made of glass, ceramics or the like, a lower metal electrode 2, a semiconductor layer 3 constituting a photoelectric conversion part, and an upper transparent electrode 4. A light receiving region is formed between the electrodes 2 and 4. However, a photocurrent is detected even when an incident light enters a region outside the light receiving region between the electrodes 2 and 4. As a result, there is a problem in that the actual light receiving region spreads, thereby deteriorating the resolution of the image sensor.
In addition, since a capacitance of the contact type image sensor having the charge storage mode is formed in the region between the confronting electrodes 2 and 4, the area of the capacitance formed by the region between the confronting electrodes 2 and 4 is small and the capacitance is limited when the image sensor is divided into bits. Therefore, there is a problem in that the image sensor is easily affected by the stray capacitance of interconnections connecting the image sensor to a driving circuit which is used for reading the document image.
In order to reduce the problems of the conventional image sensor, an image sensor having a structure shown in FIG. 2 has been proposed in a Japanese Laid-Open Patent Application No. 61-82570, for example. In FIG. 2, those parts which are essentially the same as those corresponding parts in FIG. 1 are designated by the same reference numerals, and a description thereof will be omitted. The proposed image sensor has a large region between the confronting electrodes 2 and 4, and a shield layer 5 is formed on the electrode 4. In this case, because the region forming the capacitance of the image sensor has the same layer structure as the photoelectric conversion part, a dark current increases by an amount corresponding to an increase in the area of the sensor part while the photocurrent does not change due to the shield layer 5 which is provided for the purpose of ensuring a certain resolution. As a result, there is a problem in that the performance of the image sensor described by a ratio (photocurrent)/(dark current) becomes deteriorated.
In addition, the part forming the capacitance uses the semiconductor constituting the photoelectric conversion part, but the voltage across the capacitance changes with time because a charge is once stored in the capacitance and discharged by the photocurrent generated responsive to the incident light when reading the document image in the charge storage mode of the image sensor. Accordingly, there is a problem in that the capacitance formed in the semiconductor changes when the voltage changes. Furthermore, because the capacitance is formed by the semiconductor, there is also a problem in that the capacitance also changes due to electrons/holes generated within the semiconductor due to the stray light.